Digital printers commonly provide a limited number of output possibilities, and are commonly binary, i.e., they produce either a dot or no dot at a given pixel location. Thus, given a color separation with 256 shades of a subtractive primary color, a set of binary printer signals is produced to approximate the continuous tone (contone) effect. This process is referred to as halftoning.
In such arrangements, over a given area and the separation having a number of contone pixels therein, each pixel value of an array of contone pixels within the area is compared to one of a set of preselected thresholds (the thresholds may be stored as a dither matrix and the repetitive pattern generated by this matrix is considered a halftone cell) as taught for example in U.S. Pat. No. 4,149,194 to Holladay, the entirety of which is incorporated herein by reference. The effect of such an arrangement is that, for an area where the image is a contone, some of the thresholds in the matrix will be exceeded, i.e., the image value at that specific location is larger than the value stored in the dither matrix for that same location, while others are not.
In the binary case, the pixels or cell elements for which the thresholds are exceeded might be printed as black or some color, while the remaining elements are allowed to remain white or uncolored, dependent on the actual physical quantity described by the data. Since the human visual system tends to average out rapidly varying spatial patterns and perceives only a spatial average of the micro-variation in spot-color produced by a printer, the halftone process described above can be used to produce a close approximation to the desired color in the contone input.
Generally, the resulting binary data is at a higher resolution relative to the input contone data. For example, an iGen3® printer made by Xerox® Corporation may receive 600×600×8 contone data from the controller and send a 4800×600×1 binary (halftoned) data to the raster output scanner (ROS). Other model engines also may expect 600×600×8 data, and the halftoning modules produce 2400×2400×1 binary patterns.
The dither matrix of threshold values is often referred to as a Holladay halftone dot or “screen,” and the process of generating the binary image from the contone image using the screen is called halftoning or “screening.”
Halftone screens are typically two-dimensional threshold arrays and are relatively small in comparison to the overall image or document to be printed. Therefore, the screening process uses an identical halftone screen repeated for each color separation in a manner similar to tiling. The output of the screening process, using a single-cell halftone dot, includes a binary pattern of multiple small arrays (i.e., “dots”), which are regularly spaced, and is determined by the size and the shape of the halftone screen. In other words, the screening output, as a two-dimensionally repeated pattern, possesses at least two fundamental spatial frequencies, which are completely defined by the geometry of the halftone screen.
Color printers, due to memory constraints, often have only a few preconfigured screens. A printer controller may have the capability to change between these screens at a page boundary or within a page on an object-tag basis. However, the controller cannot configure the engine to use a different screen that may be more appropriate for a particular application. Thus, the user is limited to use the predefined image screens, even though another screen (not predefined) may be more appropriate for the imaging application. As a result, the image rendering may be sub-optimal.